Communication recovering system for wavelength division multiplexed passive optical network

ABSTRACT

A communication recovering system for a wavelength division multiplexed passive optical network (WDM PON). The communication recovering system can recover fault of optical fibers between the central office and the remote nodes without additional optical fibers by grouping two remote nodes and employing AWGs having periodic transmission characteristics, and can also simply and rapidly recover such a fault with minimal optical loss using 1×N structure of the AWGs and On-Off optical switches, although protection optical fibers are additionally installed therein. The communication recovering system has advantages in that it can simplify network structure, be cost-effectively implemented, reduce optical loss, and rapidly perform protection of optical fiber fault.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication recovering system, andmore particularly to a communication recovering system for a wavelengthdivision multiplexed passive optical network (WDM PON) which is capableof being implemented with only a single optical fiber using aperiodically arrayed waveguide grating (AWG) or protecting/recoveringfault of optical fiber by minimizing optical fiber loss, although doubleoptical fibers are used.

2. Description of the Related Art

Recently, as various data services and multimedia services are rapidlyincreased through the Internet, a large amount of transmission capacityis needed in a subscriber network. In order to comply with suchrequirement, a wavelength division multiplexed passive optical network(WDM PON) has attracted considerable attention. Here, the WDM PON iscapable of providing a wide variety of services as optical signals whosewavelengths are different from each other are provided to eachsubscriber, and a plurality of optical signals are implemented via wavedivision multiplexing to be transmitted using a single optical fiber.Such a WDM PON has advantages in that, as outdoor networks areimplemented with passive elements to comply with fast transmission ofinformation, networks can be easily installed, maintained andadministered, and also extension and security are high. Furthermore, theWDM PON can provide various kinds of services according to wavelengths.

Although such a WDM PON has the above advantages, when substantiallyimplementing the WDM PON, fist of all, economical efficiency thereformust be considered. For that, various light sources, such as, awavelength-selectivity distributed feedback laser, an injection-lockedFabry-Perot laser, and a spectrum-sliced light emitting diode, etc.,have been researched.

Another consideration factor is network reliability when the WDM PON issubstantially implemented. Since the WDM PON has a high transmissionspeed for optical signals compared with prior art subscriber networks,when communication outage occurs due to cutting or errors of opticalfibers or optical components, etc., effects thereof are very serious.Generally, when optical components are installed in a sealed containeror stable place, occurrence of errors can be reduced, or when the samecomponents are additionally positioned therein, errors can be moreeffectively prevented. However, since the optical fibers are constructedas outdoor networks, they may be cut at any place and at any time byunexpected causes, such as, road construction, pipe works, and groundcollapse, etc. Especially, when a wavelength division multiplexedoptical subscriber network suffers structural breakdown, it isimpossible to recover the network, thereby weakening reliability of theoptical subscriber network.

FIG. 1 a is a view illustrating a system for a wavelength divisionmultiplexed passive optical network (WDM PON) of a double stararchitecture of the prior art.

Referring to FIG. 1 a, a central office, CO, is connected to a pluralityof remote nodes, RN, through one or more optical fibers. Optical signalsare divided into different wavelengths in each remote node including apassive wavelength division demultiplexer, and then are transmitted toeach optical network unit, ONU, through optical fibers.

The central office includes transmitters, receivers, wavelengthmultiplexers and demultiplexers. Optical signals whose wavelengths aredifferent from each other are multiplexed by arrayed waveguide gratings(AWG) and are then transmitted to remote notes via optical fibers. Here,the optical signals are transmitted from a plurality of transmitters.Another AWG in the remote node demulplexes multiplexed signals to sendoptical signals whose wavelengths are different from each other to eachoptical network unit. A receiver in the optical network unit transformsthe optical signals into electrical signals to perform communication.

Conversely, optical signals transmitted from the transmitters in theoptical network units are multiplexed by the same AWG in the remotenodes, and are then transmitted to upstream. Afterwards, the transmittedsignals are demultiplexed in the central office and are then received bythe receiver.

Accordingly, since a plurality of multiplexed WDM signals aresimultaneously transmitted between the central office and the remotenodes, there may be risk that a plurality of optical signals are lostdue to optical fiber fault, which is referred to as communicationoutage. Also, since only a few of specific wavelength optical signalsare transmitted between the remote nodes and optical network units,there may be risk that the specific wavelength signals are lost due tooptical fiber fault. In order to prevent such optical fiber faults,another optical fiber installed at a detouring optical fiber must besecured. When installing the detouring optical fiber, economicalefficiency, efficiency, network recovery time, etc. must be considered,and also the network must be designed such that loss of optical signalsis minimized.

FIG. 1 b is a view illustrating an optical fiber doubling technology torecover optical fiber fault between a central office and remote nodes inthe system of the WDM PON according to FIG. 1 a.

Referring to FIG. 1 b, a plurality of transmitters Tx and receivers Rxin a central office CO perform communication with transmitters andreceivers in each subscriber based on optical signals which areallocated unique wavelengths. The optical fibers having a plurality ofwavelengths perform wavelength division multiplexing or demultiplexingvia an AWG having a 1×N structure in the central office and the remotenodes. Here, the AWG in the central office is connected to a 1×2 opticalswitch (1×2 OS), and the AWG in the remote node is connected to a 1×2star coupler. The 1×2 OS and the 1×2 star coupler are connected to aworking fiber and a protection fiber, respectively. Here, the workingfiber is operated in a normal state. The protection fiber is operated ina state wherein the working fiber is not operated, and installed thereinto form a detouring path. Therefore, the 1×2 OS is connected to theworking fiber in a normal state to perform communication between eachoptical network unit and the central office. When there is a fault atthe working fiber, an optical fiber fault monitor, M, detects the faultand rapidly changes the state of the 1×2 OS such that currentcommunication is maintained through the protection fiber. After changingthe state of the optical switch, optical signals are recovered throughthe protection fiber such that communication can be maintained withoutstoppage. Although such an optical fiber doubling technology requires anadditional optical fiber to form a detouring path, it has stillattracted considerable attention since any fault generated in theoptical fiber can be rapidly recovered. Also, as the remote nodes areimplemented with passive elements, costs for maintaining and repairingthe networks can be minimized.

However, when the method of FIG. 1 b is applied thereto, it has adisadvantage in that performance margin of signals is reduced by morethan 3 dB due to optical loss of the star coupler. Also, it hasdrawbacks in that fault of optical fiber between the remote nodes andthe optical network units cannot be recovered. [REFERENCE: A. J.Phillips et al., “Redundancy strategies for a high splitting opticallyamplified passive optical network,” J. Lightwave Technol., February2001]

On the other hand, although specific methods to implement such anoptical fiber doubling technology have been proposed [REFERENCE: A. H.Gnauck, et al., “Reliable architecture for fiber-based broadband localaccess networks,” US Statutory invention registration, US H2075H, Aug,5, 2003], when a network is entirely connected using optical fibers,since a plurality of protection fibers and AWGs are required, thenetworks are extremely complicated. Also the networks are notcost-effective. On the other hand, when the number of AWGs is reduced,since usage efficiency for leads of the AWG lead becomes less than 50%and signals are separated using a star coupler, it has disadvantages inthat loss of the optical fiber is largely increased. Accordingly,optical fiber protection devices in the WDM PON, which are capable ofcost-effectively and efficiently using the networks and minimizingoptical fiber loss, are needed.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide awavelength division multiplexed passive optical network (WDM PON) whichis capable of minimizing the number of additional elements and opticalfibers needed to enhance economical efficiency and efficiency,minimizing optical loss to maximize margin of optical signals, anddetecting fault of optical fibers to rapidly recover the optical fibers.

In accordance with first aspect of the present invention, the above andother objects can be accomplished by the provision of a communicationrecovering system for a wavelength division multiplexed passive opticalnetwork (WDM PON) comprising: a central office, remote nodes, andoptical network units.

The central office includes cyclic arrayed waveguide gratings (AWGs) C1and C2; a first 1×2 optical switch which is installed at multiplexedsignal input and output leads of the cyclic AWG C1; a second 1×2 opticalswitch which is installed at multiplexed signal input and output leadsof the cyclic AWG C2; a blue/red (B/R) band coupler C1 which isconnected to each of the first and the second 1×2 optical switches; aB/R band coupler C2 which is connected to each of the first and second1×2 optical switches; a first optical fiber fault monitor which isinstalled on an optical fiber connecting the first 1×2 optical switch tothe B/R band coupler C1, and connected to the first 1×2 optical switch;and a second optical fiber fault monitor which is installed on anoptical fiber connecting the second 1×2 optical switch to the B/R bandcoupler C2, and connected to the second 1×2 optical switch.

The remote nodes include a first remote node RN1 which has a firstcyclic AWG R1 whose multiplexed signal input/output lead is connected tothe B/R band coupler C1 and whose demultiplexed signal input/outputleads are connected to B/R band couplers R1, respectively; and a secondremote node RN2 which has a second cyclic AWG R2 whose input/outputleads are connected to the B/R band coupler C2 and whose demultiplexedsignal input/output leads are connected to B/R band couplers R2,respectively.

The optical network units include a plurality of transmitters/receivers;and star couplers which connect the plurality of transmitters/receiversto the B/R band couplers R1 and R2.

Here, when there is a fault in an optical fiber connecting the B/R bandcoupler C1 to the cyclic AWG R1 or at an optical fiber connecting theB/R band coupler R1 to the star coupler of the ONU, optical signals areinputted/outputted thereto/therefrom through the first 1×2 opticalswitch and the B/R band coupler C2, and when there is a fault in anoptical fiber connecting the B/R band coupler C2 to the cyclic AWG R2 orat an optical fiber connecting the B/R band coupler R2 to the starcoupler of the ONU, optical signals are inputted/outputtedthereto/therefrom through the second 1×2 optical switch and the B/R bandcoupler C1.

In accordance with a second aspect of the present invention, there isprovided a communication recovering system for a wavelength divisionmultiplexed passive optical network (WDM PON) comprising: a centraloffice, remote nodes, and optical network units.

The central office includes cyclic arrayed waveguide gratings (AWGs) C1and C2; a first 1×2 star coupler installed at the multiplexed signalinput/output leads of the cyclic AWG C1; a second 1×2 star couplerinstalled at the multiplexed signal input/output leads of the cyclic AWGC2; a blue/red band coupler C1 connected to each of the first and secondstar couplers; and a blue/red band coupler C2 connected to each of thefirst and second star coupler.

The remote nodes include a first remote node RN1 which has a firstcyclic AWG R1 whose multiplexed signal input/output lead is connected tothe B/R band coupler C1 and whose demultiplexed signal input/outputleads are connected to B/R band couplers R1, respectively; and a secondremote node RN2 which has a second cyclic AWG R2 whose input/outputleads are connected to the B/R band coupler C2 and whose demultiplexedsignal input/output leads are connected to B/R band couplers R2,respectively.

The optical network units include a 1×2 optical switch connecting theblue/red band couplers R1 or R2 to transmitter/receiver; and an opticalfiber fault monitor which is installed at an optical fiber connectingthe 1×2 optical switch to the blue/red band coupler R1 or R2, andconnected to the 1×2 optical switch.

Here, when there is a fault in an optical fiber connecting the B/R bandcoupler C1 to the cyclic AWG R1 or at an optical fiber connecting theB/R band coupler R1 to the 1×2 optical switch, optical signals areinputted/outputted thereto/therefrom through the B/R band couplers C2and R2 as the B/R band coupler R2 is connected to the 1×2 opticalswitch, and when there is a fault in an optical fiber connecting the B/Rband coupler C2 to the cyclic AWG R2 or at an optical fiber connectingthe B/R band coupler R2 to the 1×2 optical switch, optical signals areinputted/outputted thereto/therefrom through the B/R band couplers C1and R1 as the B/R band coupler is connected to the 1×2 optical switch.

In accordance with a third aspect of the present invention, there isprovided a communication recovering system for wavelength divisionmultiplexed passive optical network (WDM PON) comprising: a centraloffice, remote nodes, and optical network units.

The central office includes cyclic arrayed waveguide gratings (AWGs) C1and C2; a first 1×2 optical switch installed at the multiplexed signalinput/output leads of the cyclic AWG C1; a second 1×2 optical switchinstalled at the multiplexed signal input/output leads of the cyclic AWGC2; a blue/red band coupler C1 connected to each of the first and secondoptical switches; a blue/red band coupler C2 connected to each of thefirst and second optical switches; a first optical fiber fault monitorwhich is installed at an optical fiber connecting the first 1×2 opticalswitch to the blue/red band coupler C1, and connected to the first 1×2optical switch; and the second optical fiber fault monitor which isinstalled at an optical fiber connecting the second 1×2 optical switchto the blue/red band coupler C2, and connected to the second 1×2 opticalswitch.

The remote nodes include a first remote node and a second remote node,wherein the first remote node includes a blue/red band coupler R11connected to the B/R band coupler C1; a B/R band coupler R12 connectedto a red lead of the B/R band coupler R11; a first 1×2 star couplerconnected to each blue lead of the B/R band couplers R11 and R12, and acyclic AWG R1 whose multiplexed signal input/output leads are connectedto the first 1×2 star coupler, and the second remote node includes a B/Rband coupler R21 connected to a B/R band coupler C2, a B/R band couplerR22 connected to a blue lead of a B/R coupler R21; a second 1×2 starcoupler connected to each of red leads of the B/R band couplers R21 andR22; and a cyclic AWG R2 whose multiplexed signal input/output leads areconnected to the second 1×2 star coupler, wherein the B/R band couplerR12 and the B/R band coupler R22 are connected to each other by opticalfibers.

Here, when there is a fault in an optical fiber connecting the B/R bandcoupler C1 to the B/R band coupler R11, optical signals areinputted/outputted thereto/therefrom through the first 1×2 opticalswitch and the B/R band coupler C2, and when there is a fault in anoptical fiber connecting the B/R band coupler C2 to the B/R band couplerR21, optical signals are inputted/outputted thereto/therefrom throughthe second 1×2 optical switch and the B/R band couplers C1.

In accordance with fourth aspect of the present invention, there isprovided a communication recovering system for a wavelength divisionmultiplexed passive optical network (WDM PON) comprising: a centraloffice, remote nodes, and optical network units.

The central office includes a cyclic 2×N AWG C1. The remote nodesinclude a cyclic 2×N AWG R1 connected to the cyclic 2×N AWG C1. Also,the optical network units is in each of the remote nodes.

Here, multiplexed signal input/output leads of the cyclic 2×N AWG C1 areconnected to those of the cyclic 2×N AWG R1 through working opticalfibers and protection optical fibers, respectively.

Here, the central office further includes On-Off optical switchesconnecting the cyclic 2×N AWG C1 to the protection optical fibers, andoptical fiber fault monitors connected to the working optical fibers andthe On-Off optical switches.

Here, when there is a fault in the working optical fibers, opticalsignals are inputted/outputted thereto/therefrom through the On-Offoptical switch and the protection optical fiber.

In accordance with a fifth aspect of the present invention, there isprovided a communication recovering system for a wavelength divisionmultiplexed passive optical network (WDM PON) comprising: a centraloffice, remote nodes, and optical network units.

The central office includes a cyclic 1×N AWG C1. The remote nodesinclude a cyclic 2×N AWG R1 connected to the cyclic 1×N AWG C1. Also,the optical network units are in each of the remote nodes.

Here, multiplexed signal input/output leads of the cyclic 1×N AWG C1 areconnected to those of the cyclic 1×N AWG R1 through working opticalfibers and protection optical fibers, respectively.

Here, the central office further includes: a 1×2 optical switchconnecting the cyclic 1×N AWG C1 to each of the working optical fibersand the protection optical fibers; an optical fiber fault monitorconnected to each of the working optical fibers and the 1×2 opticalswitch; and an electrical switch array which is installed at front endsof data input/output leads of transmitters and receivers, which areinstalled at multiplexed signal input/output leads of the cyclic 1×N AWGC1, such that, when a fault occurs in the working optical fibers andthen the cyclic 1×N AWG C1 is connected to the protection optical fiber,the electrical switch array can simultaneously change input/output pathsof data inputted/outputted to/from the transmitters and receivers tochannels therebeside according to monitoring signals of the opticalfiber fault monitor.

In accordance with a sixth aspect of the present invention, there isprovided a communication recovering system for a wavelength divisionmultiplexed passive optical network (WDM PON) comprising: a centraloffice, remote nodes, and optical network units.

The central office includes at least one arrayed waveguide grating(AWG).

The remote node includes the AWG which is installed corresponding to theAWG of the central office one-to-one; and

The optical network units include a transmitter and receiver, which areconnected to the AWG of the remote node, wherein the transmitter andreceiver of each of the optical network units are connected to the AWGof the remote node through a 2×2 optical switch.

Here, a transceiver connected to the transmitter and the receiver isinstalled in n-th demultiplxed signal input/output lead of the AWG ofthe remote node.

Here, an n-x-th 2×2 optical switch installs the AWG of the remote nodein the first input/output lead thereof, and installs n-x-th transmitterand receiver in the second input/output leads thereof, which areoptically connected to the first input/output lead, in which third andfourth input/output leads of the n-x-th 2×2 optical switch are connectedto fourth and third input/output leads of a n-x−1-th and n-x+1-th 2×2optical switches, respectively, wherein an n-1-th 2×2 optical switchinstalls the transceiver in the fourth input/output lead thereof.

Here, the AWG of the remote node and the 2×2 optical switches installoptical fiber fault monitors therein, which are connected to opticalfibers connecting each of the first input/output leads to each of the2×2 optical switches.

Here, when a fault occurs in any one of the optical fibers, the 2×2optical switch connected to the optical fiber having such a fault isswitched, such that the transmitter and receiver in the optical fiberhaving the fault can be connected to the transceiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 a is a view illustrating a system for a wavelength divisionmultiplexed passive optical network (WDM PON) of a double stararchitecture of the prior art;

FIG. 1 b is a view illustrating an optical fiber doubling technology torecover an optical fiber fault between a central office and remote nodesin the system of the WDM PON according to FIG. 1 a;

FIG. 2 a is a view describing a communication recovering system in a WDMPON according to a first embodiment of the present invention;

FIG. 2 b is a view describing characteristics of a blue/red band couplerin a communication recovering system in a WDM PON of FIG. 2 a;

FIG. 2 c is a view illustrating a modification of the communicationrecovering system in a WDM PON according to the first embodiment of thepresent invention;

FIG. 2 d is a view illustrating another modification of thecommunication recovering system in a WDM PON according to the firstembodiment of the present invention;

FIG. 3 a to FIG. 3 c are views describing a communication recoveringsystem in a WDM PON according to a fourth embodiment of the presentinvention;

FIG. 3 d is a view illustrating a modification of the communicationrecovering system in a WDM PON according to the fourth embodiment of thepresent invention;

FIG. 4 a is a view describing a communication recovering system in a WDMPON according to a sixth embodiment of the present invention; and

FIG. 4 b is a view illustrating a modification of the communicationrecovering system in a WDM PON according to the sixth embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the attached drawings, preferred embodiments of the presentinvention are described in detail below.

The present invention is related to a system that can automaticallyrecover communication when communication fault occurs due to opticalfibers as configurations of a central office and remote nodes and/oroptical network units in a wavelength division multiplexed passiveoptical network (WDM PON), which includes the central office, the remotenodes, and optical network units.

Embodiment 1

FIG. 2 a is a view describing a communication recovering system in a WDMPON according to a first embodiment of the present invention, and FIG. 2b is a view describing characteristics of blue/red band coupler in acommunication recovering system in a WDM PON of FIG. 2 a.

Referring to FIG. 2 a, a central office CO in the communicationrecovering system includes; a first cyclic AWG (hereinafter, cyclic AWGC1) and a second cyclic AWG (herein after cyclic AWG C2) each of whichhas a 1×N structure and a cyclic pass characteristic; a first 1×2optical switch (hereinafter, first 1×2 OS) which is installed atmultiplexed signal input and output leads of the cyclic AWG C1 and has a1×2 structure; a second 1×2 optical switch (hereinafter, second 1×2 OS)which is installed at multiplexed signal input and output leads of thecyclic AWG C2; a first blue/red band coupler (hereinafter, B/R bandcoupler C1) which is connected to each of the 1×2 optical switches; asecond blue/red band coupler, C2 B/R, which is connected to each of thefirst and second 1×2 OSs; a first optical fiber fault monitor, M1, whichis installed on an optical fiber connecting the first 1×2 OS to the B/Rband coupler C1, and connected to the first 1×2 OS; and a second opticalfiber fault monitor, M2, which is installed on an optical fiberconnecting the second 1×2 OS to the B/R band coupler C2, and connectedto the second 1×2 OS. Here, the first 1×2 OS connects the cyclic AWG C1to the B/R band coupler C1 or the B/R band coupler C2 according tosignals outputted from the first optical fiber fault monitor M1. Also,the second 1×2 OS connects the cyclic AWG C2 to the B/R band coupler C1or the B/R band coupler C2 according to signals outputted from thesecond optical fiber fault monitor M2.

The remote node includes: a first remote node RN1 which has a firstcyclic AWG (hereinafter cyclic AWG R1) whose input and output leads areconnected to the B/R band coupler C1; and a second remote node RN2 whichhas a second cyclic AWG (hereinafter cyclic AWG R2) whose input/outputleads are connected to the B/R band coupler C2. Here, in the cyclic AWGR1 and cyclic AWG R2, demultiplexed signal input/output leads areconnected to a blue/red coupler (hereinafter B/R R1) and to a blue/redcoupler (hereinafter B/R R2), respectively.

Through star couplers, each of transmitter/receiver in the ONU isconnected to one of among B/R couplers, which are connected todemuliplexed signal input/output leads of the cyclic AWG R1 and to oneof among B/R couplers, which are connected to demultiplexed signalinput/output leads of the cyclic AWG R2.

With reference to FIG. 2 b, the B/R band coupler serves to performwavelength division multiplexing and demultiplexing for upstream anddownstream signals of the first ONU, which performstransmission/reception of optical signals in blue band wavelength, andfor upstream and downstream signals of the second ONU, which performstransmission/reception of optical signals in red band wavelength.

For example, assuming a case where there is no fault on an optical fiberof the WDM PON. When the cyclic AWG C1 multiplexes downstream opticalsignals of blue band wavelength and outputs, the optical signals areinputted to the cyclic AWG R1 via the first 1×2 OS and the first cyclicB/R C1. The cyclic AWG R1 demultiplexes the inputted optical signalsbased on wavelength. Each demultiplexed signal is transmitted to the ONUin which transmitters/receivers are installed, via a lead of the B/Rband coupler passing only blue wavelength therethrough, which is calleda blue lead. The upstream optical signal outputted from the ONU isinputted into the cyclic AWG R1 through the B/R band coupler. Theoptical signals inputted into the cyclic AWG R1 are multiplexed withother optical signals of blue band wavelength, which are outputted fromthe other ONU. The multiplexed signals are inputted to the cyclic AWG C1through the B/R C1.

Similarly, downstream optical signals of red band wavelength, which aretransmitted via the cyclic AWG C2, the second 1×2 OS and the secondcyclic B/R C2, are inputted to the ONU in which transmitters/receiversare installed, via a lead of the B/R band coupler passing only redwavelength therethrough together with the cyclic AWG R2, which is calleda red lead. The upstream optical signal outputted from the ONU isinputted into the cyclic AWG R2 through the B/R band coupler, the cyclicAWG R2 and the second B/R C2.

When there is a fault in an optical fiber connecting the B/R bandcoupler C1 to the cyclic AWG R1 or in an optical fiber connecting theblue leads of the B/R band coupler R1 to the subscriber using blue bandwavelength, the first optical fiber fault monitor installed at thecentral office monitors optical power of the upstream optical signal todetect fault of the optical fiber, and controls the first 1×2 OS suchthat the downstream optical signals of blue band wavelength which aremultiplexed at the cyclic AWG C1 can be transmitted via the first 1×2 OSand the second B/R C2. Therefore, the downstream optical signals of blueband wavelength multiplexed in the cyclic AWG C1 and the downstreamoptical signals of red band wavelength multiplexed in the cyclic AWG C2are multiplexed by the second B/R C2, and are then inputted to thecyclic AWG R2 to be demultiplexed. The demultiplexed signals areinputted to the ONU, which uses blue band, through the blue lead of theB/R band coupler and the ONU, which uses red band, through the red leadof the B/R band, respectively. Upstream optical signals outputted fromthe two ONUs are inputted to the cyclic AWG R2 through the 1×2 starcoupler located at the ONU and the B/R band coupler located at theremote node R2. The optical signals inputted to the cyclic AWG R2 aremultiplexed together with the optical signals of blue/red bandwavelengths, which are outputted from the other ONUS, and are thenoutputted to the B/R C2. Namely, the optical signals of blue bandwavelength from the cyclic AWG R2 are inputted to the cyclic AWG C2through the B/R C2, and the optical signals of red band wavelength areinputted to the cyclic AWG C2.

When there is a fault in the optical fiber connecting the B/R C2 to thecyclic AWG R2 or at the optical fiber connecting the red leads of thefirst B/R R2 to the subscriber using red band wavelength, the secondoptical fiber fault monitor installed at the central office monitors andcontrols the second 1×2 OS such that downstream optical signals of redband wavelength, which are multiplexed in the cyclic AWG C2, aretransmitted thereto through the second 1×2 OS and the B/R C1. Therefore,sine the situation wherein the fault occurs at the optical fiberconnecting the B/R C1 to the cyclic AWG R1 is similar to that of thefault where the fault occurs at the optical fiber connection the bluelead of the B/R R1 to the subscribers using blue band wavelength exceptthat the optical signals are transmitted/received therefrom/theretothrough the B/R C1 and the cyclic AWG R1, the above mentioned proceduresafter faults are identically proceeded.

In the first embodiment of the present invention, the upstream opticalsignals in blue wavelength band and the upstream optical signals in redwavelength band are worked by a distance of Free Spectral Range (FSR) asthe period of the cyclic AWG having a cyclic passing characteristic, andthe optical signals in red wavelength band and blue wavelength band aredivided or coupled, according to the B/R band coupler, and are thentransmitted thereto. Therefore, the optical signals are prevented frominterfering with each other. Also, the ONUs use only wavelengths in theblue band or only wavelengths in the red band, such that optical signalscannot interfere with one another.

Embodiment 2

FIG. 2 c is a view illustrating a modification of the communicationrecovering system in a WDM PON according to the first embodiment of thepresent invention.

Referring to FIG. 2 c associated with FIG. 2 a, 1×2 star couplers areinstalled in the central office CO according to the present invention,instead of the 1×2 optical switches as shown in FIG. 2 a, but theoptical fiber fault monitors are not installed therein, which isdifferent from FIG. 2 a. On the other hand, the ONUs according to thepresent invention install an 1×2 optical switch, instead of the starcouplers as shown in FIG. 2 a. Also, the ONUs install optical fibersconnecting the 1×2 optical switch to the B/R band coupler R1 or the B/Rband coupler, and an optical fiber fault monitor to be connected to the1×2 optical switch.

Therefore, when there is a fault in the optical fiber connecting the B/RC1 band coupler to the cyclic AWG R1, or in the optical fiber connectingthe B/R band coupler R1 to the 1×2 optical switch, the B/R R2 bandcoupler and the 1×2 optical switch are connected to each other, suchthat optical signals can be inputted or outputted thereto/therefrom,through the B/R C2 band coupler and the B/R R2 band coupler.

Also, when there is any fault in the optical fiber connecting the B/R C2band coupler to the cyclic AWG R2, or in the optical fiber connectingthe B/R band coupler R2 to the 1×2 optical switch, the B/R R1 bandcoupler and the 1×2 optical switch are connected to each other, suchthat optical signals can be inputted or outputted therein/thereto,through the B/R C1 band coupler and the B/R R1 band coupler.

Embodiment 3

FIG. 2 d is a view illustrating another modification of thecommunication recovering system in a WDM PON according to the firstembodiment of the present invention.

Referring to FIG. 2 d associated with FIG. 2 a, the first remote nodeRN1 includes: a B/R band coupler R11 connected to a B/R band coupler C1;a B/R band coupler R12 connected to a lead of a B/R coupler R11; a first1×2 star coupler connected to each blue lead of the B/R band couplersR11 and R12; and a cyclic AWG R1 whose multiplexed signal input/outputleads are connected to the first 1×2 star coupler. The second remotenode RN2 includes: a B/R band coupler R21 connected to a B/R bandcoupler C2; a B/R band coupler R22 connected to a blue lead of a B/Rcoupler R21; a second 1×2 star coupler connected to each of red leads ofthe B/R band couplers R21 and R22; and a cyclic AWG R2 whose multiplexedsignal input/output leads are connected to the second 1×2 star coupler.Here, the B/R band coupler R12 and the B/R band coupler R22 areconnected to each other by optical fibers.

Each transmitter/receiver of the ONU is directly connected to thedemultiplexed signal input/output leads of the cyclic AWG R1 or thecyclic AWG R2.

Accordingly, in case that there is no fault in the optical fiber of theWDM PON, when the cyclic AWG C1 multiplexes downstream optical signalsof blue band wavelength and then outputs them, the optical signals passthrough the B/R band coupler and the first 1×2 star coupler via thefirst 1×2 optical switch and the B/R band coupler C1 to be inputted intothe cyclic AWG R1. Then, the cyclic AWG R1 demultiplexes the inputtedsignals based on wavelengths such that each demultiplexed signal can beinputted to the ONU. Upstream optical signals outputted from the ONU areinputted into cyclic AWG R1. The optical signals inputted into thecyclic AWG R1 are multiplexed with optical signals having blue bandwavelength which are outputted from other ONUs, and are then outputted.The outputted signals pass through the first 1×2 star coupler and theB/R band coupler R11. After that, the signals are inputted into cyclicAWG C1 through the B/R band coupler C1. Similarly, the downstreamoptical signals of red band wavelength, transmitted through the cyclicAWG C2, the second 1×2 optical switch and the B/R band coupler C2, passthrough the B/R band coupler R21 and the second 1×2 star coupler, andare then inputted to an ONU via the cyclic AWG R2. Upstream opticalsignals outputted from the ONU are inputted to the cyclic AWG C2 throughthe cyclic AWG R2, the second 1×2 star coupler and B/R band coupler R21,and the B/R band coupler C2.

On the other hand, when there is a fault in the optical fiber connectinga B/R band coupler C1 with the B/R band coupler R11, the first opticalfiber fault monitor installed in the central office monitors opticalpower of upstream optical signals to detect whether there is any faultin the optical fiber, and controls the first 1×2 optical switch suchthat the downstream optical signals of blue band wavelength, which aremultiplexed in the cyclic AWG C1, are transmitted through the first 1×2optical switch and the B/R band coupler C2. Therefore, the downstreamoptical signals of blue band wavelength, which are multiplexed in thecyclic AWG C1, and the downstream optical signals of red bandwavelength, which are multiplexed in the cyclic AWG C2, are multiplexedin the B/R band coupler C2, and are then inputted to the B/R bandcoupler R21. After that, the optical signals inputted to the B/R bandcoupler are demultiplexed into blue band wavelength signals and red bandwavelength signals, respectively. The blue band wavelength signals areinputted to the cyclic AWG R1 through the B/R band couplers R22 and R12and the first 1×2 star coupler to be demultiplexed therein. After that,the demultiplexed signals are inputted to ONUs, respectively. Theupstream optical signals outputted from each ONU are inputted to thecyclic AWG R1 to be multiplexed, and are then inputted to the B/R bandcoupler through the first 1×2 star coupler and the B/R band coupler R12and R22. After that, the upstream optical signals are multiplexed withred and blue wavelength signals in the B/R band coupler R12 to beoutputted therefrom. Of the outputted optical signals from the B/R bandcoupler R12, optical signals of blue band wavelength are inputted to thecyclic AWG C1 through the B/R band coupler C2, and the optical signalsof red band wavelength are inputted to the cyclic AWG C2.

Also, in case that there is a fault in the optical fiber connecting aB/R band coupler C2 with B/R band coupler R21, the second optical fiberfault monitor installed in the central office monitors such fault, andcontrols the second 1×2 optical switch such that downstream opticalsignals of red band wavelength, multiplexed in the cyclic AWG C2, can betransmitted through the second 1×2 optical switch and B/R band couplerC1. Therefore, the situation, where there is a fault in the opticalfiber connecting a B/R band coupler C2 with B/R band coupler R21, is thesame as when there is a fault in the optical fiber connecting a B/R bandcoupler C1 with B/R band coupler R11, except that the optical signalsare transmitted/received through the B/R band coupler C1 and the B/Rband coupler R11.

When communication is interrupted as a fault occurs in any opticalfiber, unlike the prior art technology where protection optical fibersare added to all of the optical fibers, since the first and secondembodiments of the present invention, as mentioned above, add theprotection optical fibers only to an optical fiber connecting the remotenode to the ONU, communication can be continuously performed through aworking optical fiber as a communication path is changed by the 1×2optical switch. Therefore, such a fault can be rapidly recovered. Also,in the case that communication is interrupted as a fault occurs in anoptical fiber connecting the central office and the remote node, unlikethe prior art technology where protection optical fibers are addedthereto, since the third embodiment of the present invention connectstwo remote nodes using a single optical fiber, communication can becontinuously performed through a working optical fiber as acommunication path is changed by the 1×2 optical switch. Therefore, sucha fault can be rapidly recovered. Consequently, compared with the priorart technology where double the number of optical fibers is used, thepresent invention can reduce maintenance costs, and rapidly recovercommunication faults because communication protection time depends onprocessing time of an optical fiber fault monitor and switching time ofa 1×2 optical switch.

Embodiment 4

FIG. 3 a to FIG. 3 c are views describing a communication recoveringsystem in a WDM PON according to a fourth embodiment of the presentinvention.

Referring to FIG. 3 a, in the communication recovering system in a WDMPON according to the present invention, a central office CO includes acyclic AWG, which is configured as a structure of 2×N and has a cyclicpass characteristic, which is called hereinafter a Cyclic AWG C1, and aremote node RN includes a cyclic 2×N AWG R1 which is connected to thecyclic 2×N AWG C1. Here, multiplexed signal input/output leads of thecyclic 2×N AWG C1 are connected to those of the cyclic 2×N AWG R1through working optical fibers and protection optical fibers,respectively. An optical fiber fault monitor M1 and an On-Off opticalswitch are installed in the central office, such that the central officeand the remote node can be connected to each other via the protectionoptical fiber when a fault occurs in the working optical fiber based onmonitoring of the working optical fiber. Namely, the central officeinstalls an On-Off optical switch and optical fiber fault monitortherein, in which the On-Off optical switch connects the cyclic 2×N AWGC1 to the protection optical fiber, and the optical fiber fault monitorM is connected to the working optical fiber to the cyclic 2×N AWG C1,respectively.

Therefore, when there is a fault in the working optical fiber, thecyclic 2×N AWG C1 is connected to the cyclic 2×N AWG R1 through theOn-Off optical switch and the protection optical fiber, such thatmultiplexed optical signals can be transmitted/receivedthereto/therefrom.

Namely, in a normal state, the On-Off optical switch is maintained in anOff-state, thereby performing upstream/downstream optical communication.Here, since the cyclic 2×N AWG periodically has a pass band at everyFSR, when each ONU uses a working optical fiber and a protection opticalfiber, it can perform communication using different wavelengths. Forexample, in case that optical network units ONU11 and ONU21 performupstream/downstream transmission using wavelengths, λ11 and λ21,respectively, λ11 and λ21 are the wavelengths that the optical networkunits ONU11 and ONU21 can be communicated to each other via protectionoptical fiber due to transmission characteristic having a 2×N structure.Therefore, in order to completely recover communication, wavelength of alight source in the transmitter must be simultaneously transformed. Forthis, the fourth embodiment of the present invention employs a lightsource for WDM PON, such as a light source employing spectrum slicing orinjection locking, which does not have wavelength selectivity. Here,when the WDM PON employs a spectrum sliced light source, since the lightsource uses a wide wavelength band and automatically allocates itswavelengths by the AWG, it can be directly used in the fourth embodimentof the present invention. On the other hand, when the WDM PON employs aFabry-Perot light source in an injection locking manner, a BroadbandLight Source (BLS) is added thereto. Also, a 1×2 optical switch isadditionally installed thereto so that the BLS is connected to theworking optical fiber or the protection optical fiber.

FIG. 3 b shows graphs based on experimental results to show whetherwavelength is transformed before and after a fault occurs in the WDM PONaccording to the present invention which uses a light source in aspectrum sliced manner.

Referring to FIG. 3 b, since wavelength is shifted by 100 GHz regardingall the channels by change of pass characteristics of the cyclic AWG,there is no inter-channel interference.

FIG. 3 c is a timing chart when the WDM PON of the fourth embodiment ofthe present invention uses a mechanical optical switch as the On-Offoptical switch.

Referring to FIG. 3 c, when a fault occurs in an optical fiber, bothoptical powers of upstream and downstream signals are reduced, therebyinterrupting communication. Here, as the optical fiber fault monitormonitors such a phenomenon and changes a state of the optical switch,all of the signals are rapidly recovered within 8 msec such that theiroptical powers can be increased. Here, protection time depends onswitching operation of the optical switch. Namely, the faster theoptical switch is operated, the shorter the protection time.

Embodiment 5

FIG. 3 d is a view illustrating a modification of the communicationrecovering system in a WDM PON according to the fourth embodiment of thepresent invention.

Referring to FIG. 3 d, in the communication recovering system in a WDMPON according to the present invention, a central office CO includes acyclic AWG, which is configured as a structure of 1×N and has a cyclicpass characteristic, which is called hereinafter a Cyclic AWG C1, and aremote node RN includes a cyclic 2×N AWG R1 which is connected to thecyclic 1×N AWG C1. Here, multiplexed signal input/output leads of thecyclic 1×N AWG C1 are connected to those of the cyclic 2×N AWG R1through working optical fibers and protection optical fibers,respectively. An optical fiber fault monitor M1 and a 1×2 optical switchare installed in the central office, such that the central office andthe remote node can be connected to each other via the protectionoptical fiber when a fault occurs in the working optical fiber based onmonitoring of the working optical fiber. Namely, a 1×2 optical switchand optical fiber fault monitor are installed in the central officeinstalls, in which the 1×2 optical switch connects the cyclic 1×N AWG C1to the protection optical fiber, and the optical fiber fault monitor M1is connected to the working optical fiber and the 1×2 optical switch,respectively.

Therefore, when there is a fault in the working optical fiber, thecyclic 1×N AWG C1 is connected to the cyclic 2×N AWG R1 through the 1×2optical switch and the protection optical fiber, such that multiplexedoptical signals can be transmitted/received thereto/therefrom. Here, anelectrical switch array is installed at front ends of data input/outputleads of the transmitters and receives, which are installed atmultiplexed signal input/output leads of the cyclic 1×N AWG C1, suchthat the electrical switch array can be connected to the transmitters,receivers and optical fibers, respectively. When a fault occurs in theworking optical fiber and thereby the cyclic 1×N AWG C1 is connected tothe protection optical fiber, the electrical switch array simultaneouslychanges input/output paths of data inputted/outputted to/from thetransmitters and receivers to channels therebeside according tomonitoring signals of the optical fiber fault monitor.

The 2×N AWG of the remote node periodically shows pass bands at everyFSR, since wavelengths of multiplexed signals of the central office arenot changed, the ONU performs communication at different wavelengthsbased on determination as to whether the central office and the remotenode are connected to each other via the working optical fiber or theprotection optical fiber. For example, under normal conditions, theoptical network units ONU11 and ONU21 perform upstream/downstreamtransmission using wavelengths λ11 and λ21, respectively, λ11 and λ21are the wavelengths at which the optical network units ONU11 and ONU21can communicate with each other via the protection optical fiber due totransmission characteristic having a 2×N structure.

Since the fourth and fifth embodiments of the present invention do notemploy star couplers, optical loss is relatively small. Also, sincenetwork protection time depends on processing time of the optical fibermonitor and switching time of an On-Off optical switch and 1×2 opticalswitch, the network is relatively rapidly recovered.

In addition, unlike the fourth embodiment of the present invention, thefifth embodiment of the present invention can relatively rapidly recovercommunication stoppage of each ONU, which is caused by a fault of theoptical fiber, although wavelength of a light source of the centraloffice is fixed. Therefore, communication can be recovered within arange of least optical loss in a network where fast communication over1.2 Gb/s is performed.

Embodiment 6

FIG. 4 a is a view describing a communication recovering system in a WDMPON according to a sixth embodiment of the present invention.

Referring to FIG. 4 a, the central office CO of the communicationrecovering system in a WDM PON includes a 1×N AWG. The remote node RNincludes a 1×N AWG which is connected to the AWG in the central office,in which demultiplexed signal input/output leads of the 1×N AWG in theremote node are transmitter and receiver of the ONUs, respectively.Especially, the n-th demultiplexed signal input/output leads of the 1×NAWG in the remote node are connected to the transceiver. Here, areserved transmitter and receiver in the n-th demultiplexed signalinput/output leads are further installed in the 1×N AWG in the central,in which each of the transmitter and receiver in each ONU is opticallyconnected to the transceiver. The transmitter and receiver of each ONUare connected to the transceiver through a 2×2 optical switch, whoseconfiguration is as below. Namely, regarding one 2×2 optical switch(hereinafter, the n-x-th 2×2 optical switch), the first input/outputlead of the n-x-th 2×2 optical switch is connected to an AWG of theremote node, the second input/output lead of the n-x-th 2×2 opticalswitch, which is optically connected to the first input/output lead, isconnected to the n-x-th transmitter and receiver, and the third andfourth input/output leads of the n-x-th 2×2 optical switch, which areoptically connected to each other, are connected to the fourth and thirdinput/output leads of the n-x−1-th and the n-x+1-th 2×2 opticalswitches, respectively. Here, the fourth input/output lead of the n−1-th2×2 optical switch, which is connected to the n−1 demultiplexed signalinput/output lead of the AWG of the remote node, is connected to atransceiver. Here, the transceiver includes a first transceiver whichinputs signals outputted from the AWG of the remote node and outputs thesignals to the ONU, and a second transceiver which inputs the signalsoutputted from the ONU and outputs the signals to the AWG of the remotenode. The input lead of the first transceiver, the output lead of thesecond transceiver, the output lead of the first transceiver, and theinput lead of the second transceiver are connected to the opticalcirculators, respectively. When the optical signals are inputted fromthe AWG of the remote node to the first transceiver, the firsttransceiver amplifies the inputted optical signals and outputs theamplified optical signals. The optical signals are inputted from the ONUto the second transceiver, the second transceiver amplifies the opticalsignals, transforms wavelength thereof, and then outputs the opticalsignals to the AWG of the remote node. Optical fiber fault monitors M'sare installed in the AWG of the remote node and the 2×2 opticalswitches, respectively, in which the optical fiber fault monitors M'smonitor optical fibers connecting the first input/output leads,respectively. The optical fiber fault monitors M's are connected to the2×2 optical switches on a one-to-one basis.

Therefore, when a fault occurs in one of the optical fibers connectingthe remote nodes to the ONUs, the optical fiber fault monitor detectssuch fault and switches the 2×2 optical switch connected to the opticalfiber having such a fault such that the transmitter and the receiverconnected to the optical fiber having the fault can be connected to thetransceiver. For example, when a fault occurs in an optical fiberconnecting the AWG of the remote node to the m-th 2×2 optical switch,the first input/output lead connected to the AWG of the remote node isconnected to the fourth input/output lead of the m−1 2×2 optical switchconnected to the third input/output lead, and the m-th transmitter andreceiver connected to the second input/output lead is connected to thethird input/output lead of the m+1-th 2×2 optical switch connected tothe fourth input/output lead, thereby being connected to thetransceiver. Therefore, when there is an fault in any one of the opticalfibers, the AWG of the remote node and the optical fibers connecting thefirst transceiver to the second transceiver are connected to thetransmitter and receiver of the ONU, which are connected to the opticalfiber having such a fault, through the are connected to at least one ofthe 2×2 optical switches. Here, since the first and second transceiversare connected to the transmitter and the receiver of the ONU through atleast one of the optical switches, the first and second transceiversamplify the inputted optical signals and output them thereto. Since theoptical signals outputted from the transmitter of the ONU connected tothe optical fiber having a fault are inputted to the reserved receiverof the central office, the second transceiver can transform wavelengthof the optical signals.

Therefore, the embodiment of the present invention can rapidly recoverfault in the optical fiber between the ONU and the remote node. Here,protection time is determined by summation of processing time of theoptical fiber monitor, switching time of the 2×2 optical switch, andoptical fiber passing time between ONUs.

The above-mentioned embodiment of the present invention may beassociated with the third to fifth embodiment of the present invention,and a transceiver within a specific area and optical transmitter andreceiver within an ONU having a fault can be communicated withrelatively little optical power.

Embodiment 7

FIG. 4 b is a view illustrating a modification of the communicationrecovering system in a WDM PON according to the sixth embodiment of thepresent invention.

The embodiment illustrated in FIG. 4 b is the same as that of FIG. 4 a,except that the first and second transceivers are connected to theoptical circulator connected to a star coupler, in which the starcoupler is connected to each of the ONUs. Since the communicationrecovering system for a wavelength division multiplexed passive opticalnetwork according to the sixth embodiment of the present invention hassufficient optical margin of optical link, when the star coupler isconnected thereto and at the same time a plurality of ONUs are seriallyor parallelly connected thereto, optical fibers between each of the ONUsand the remote node can be protected. Therefore, the embodiment of thepresent invention can protect optical fibers in a one to N mannerregarding a WDM PON in which each remote node shares N ONUs, and also ina one to nN manner regarding a WDM PON in which a plurality of remotenodes shares nN ONUs, in which n remote nodes are shared.

As apparent from the above description, the communication recoveringsystem for a WDM PON according to the present invention has advantagesin that, cyclic transmission characteristics of the cyclic AWG are used,network efficiency is high by grouping two remote nodes in the WDM PON,recover is relatively rapidly achieved, and network maintenance costsare reduced as an outdoor network is configured by passive elements.

Also, the communication recovering system for a WDM PON according to thepresent invention has advantages in that, optical loss can be minimizedin the WDM PON employing a light source without wavelength selectivitywhen an AWG of 2×N structure is used and in the WDM PON employing alight source whose wavelength is constant when electrical switchingarray is used, the system is cost-effective since the number of elementsfor implement can be reduced, and recover can be relatively rapidlyachieved.

In addition, the communication recovering system for a WDM PON accordingto the present invention has advantages in that, since preservedtransmitter and receiver, and transceiver are additionally used, networkefficiency is increased, fault in the optical fibers between the remotenode and the ONUs can be cost-effectively and rapidly recovered whileoptical margin is sufficiently secured.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A communication recovering system for wavelength division amultiplexed passive optical network (WDM PON) comprising: a centraloffice including: cyclic arrayed waveguide gratings (AWGs) C1 and C2; afirst 1×2 optical switch which is installed at a multiplexed signalinput and output lead of the cyclic AWG C1; a second 1×2 optical switchwhich is installed at a multiplexed signal input and output lead of thecyclic AWG C2; a first blue/red (B/R) band coupler which is connected toeach of the first and the second 1×2 optical switches; a second B/R bandcoupler which is connected to each of the first and second 1×2 opticalswitches; a first optical fiber monitor which is installed on an opticalfiber connecting the first 1×2 optical switch to the first B/R bandcoupler, and is connected to the first 1×2 optical switch; and a secondoptical fiber fault monitor which is installed on an optical fiberconnecting the second 1×2 optical switch to the second B/R band coupler,and is connected to the second 1×2 optical switch; remote nodesincluding: a first remote node RN1 which has a first cyclic AWG R1 whosemultiplexed signal input/output lead is connected to the first B/R bandcoupler and whose demultiplexed signal input/output leads are connectedto first remote B/R band couplers, respectively; and a second remotenode RN2 which has a second cyclic AWG R2 whose input/output lead isconnected to the second B/R band coupler and whose demultiplexed signalinput/output leads are connected to second remote B/R band couplers,respectively; and optical network units including: a plurality oftransmitters/receivers; and star couplers which connect the plurality oftransmitters/receivers to the first remote and second remote B/R bandcouplers, wherein when there in an fault in an optical fiber connectingthe first B/R band coupler to the cyclic AWG R1 or at an optical fiberconnecting the first remote B/R band coupler to a corresponding starcoupler of a corresponding ONU, optical signals are inputted/outputtedthrough the first 1×2 optical switch and the second B/R band coupler,and when there is a fault in an optical fiber connecting the second B/Rband coupler C2 to the cyclic AWG R2 or in an optical fiber connectingthe second remote B/R band coupler to a corresponding star coupler of acorresponding ONU, optical signals are inputted/outputted through thesecond 1×2 optical switch and the first B/R band coupler.
 2. Acommunication recovering system for a wavelength division multiplexedpassive optical network (WDM PON) comprising: a central officeincluding: cyclic arrayed waveguide gratings (AWGs) C1 and C2; a first1×2 star coupler installed at a multiplexed signal input/output lead ofthe cyclic AWG C1; a second 1×2 star coupler installed at a multiplexedsignal input/output lead of the cyclic AWG C2; a first blue/red bandcoupler connected to each of the first and second star couplers; and asecond blue/red band coupler connected to each of the first and secondstar couplers; remote nodes including: a first remote node RN1 which hasa first cyclic AWG R1 whose multiplexed signal input/output lead isconnected to the first B/R band coupler and whose demultiplexed signalinput/output leads are connected to first remote B/R band couplers,respectively; and a second remote node RN2 which has a second cyclic AWGR2 whose input/output lead is connected to the second B/R band couplerand whose demultiplexed signal input/output leads are connected tosecond remote B/R band couplers, respectively; and optical network unitsincluding: 1×2 optical switches connecting the first remote and secondremote blue/red band couplers and to transmitters/receivers; and foreach optical network unit an optical fiber fault monitor which isinstalled at an optical fiber connecting a corresponding 1×2 opticalswitch to the corresponding first remote blue/red band coupler or thecorresponding second remote blue/red band coupler, and is connected tothe 1×2 optical switch, wherein when there is a fault in an opticalfiber connecting the first B/R band coupler to the cyclic AWG R1 or atan optical fiber connecting the corresponding first remote B/R bandcoupler to the 1×2 optical switch, optical signals areinputted/outputted through the second B/R band coupler and correspondingsecond remote B/R band coupler as the corresponding second remote B/Rband coupler is connected to the 1×2 optical switch, and when there is afault in an optical fiber connecting the second B/R band coupler to thecyclic AWG R2 or at an optical fiber connecting the corresponding secondremote B/R band coupler to the 1×2 optical switch, optical signals areinputted/outputted through the first B/R band coupler and correspondingfirst remote B/R band coupler as the corresponding first remote B/R bandcoupler is connected to the 1×2 optical switch.